Make Page an interface

The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue  #5074, "pages from other data sources".

But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.

Most notable changes:

* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
  This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
  This improves the taxonomy template `.Title` situation and make common template constructs much simpler.

See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778

resources/resource/resource_helpers.go

@@ -0,0 +1,70 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package resource
+
+import (
+	"strings"
+	"time"
+
+	"github.com/gohugoio/hugo/helpers"
+
+	"github.com/spf13/cast"
+)
+
+// GetParam will return the param with the given key from the Resource,
+// nil if not found.
+func GetParam(r Resource, key string) interface{} {
+	return getParam(r, key, false)
+}
+
+// GetParamToLower is the same as GetParam but it will lower case any string
+// result, including string slices.
+func GetParamToLower(r Resource, key string) interface{} {
+	return getParam(r, key, true)
+}
+
+func getParam(r Resource, key string, stringToLower bool) interface{} {
+	v := r.Params()[strings.ToLower(key)]
+
+	if v == nil {
+		return nil
+	}
+
+	switch val := v.(type) {
+	case bool:
+		return val
+	case string:
+		if stringToLower {
+			return strings.ToLower(val)
+		}
+		return val
+	case int64, int32, int16, int8, int:
+		return cast.ToInt(v)
+	case float64, float32:
+		return cast.ToFloat64(v)
+	case time.Time:
+		return val
+	case []string:
+		if stringToLower {
+			return helpers.SliceToLower(val)
+		}
+		return v
+	case map[string]interface{}: // JSON and TOML
+		return v
+	case map[interface{}]interface{}: // YAML
+		return v
+	}
+
+	return nil
+}